The present invention relates to an energy cable having a cold-strippable semiconductive layer. More particularly, the present invention relates to an energy cable having a thermoplastic insulating layer in contact with a cold-strippable semiconductive outer layer.
Cables for transporting electric energy generally include at least one cable core. The cable core is usually formed by at least one conductor sequentially covered by an inner polymeric layer having semiconducting properties, an intermediate polymeric layer having electrically insulating properties, an outer polymeric layer having semiconducting properties. Cables for transporting medium or high voltage electric energy generally include at least one cable core surrounded by at least one screen layer, typically made of metal or of metal and polymeric material. The screen layer can be made in the form of wires (braids), of a tape helically wound around the cable core or a sheet longitudinally wrapped around the cable core.
The polymeric layers surrounding the conductor/s are commonly made from a polyolefin-based crosslinked polymer, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also crosslinked, as disclosed, e.g., in WO 98/52197. The crosslinking step, carried out after extruding the polymeric material onto the conductor, gives the material satisfactory mechanical and electrical properties even under high temperatures both during continuous use and with current overload.
For various reasons including to address requirements for materials which should not be harmful to the environment both during production and during use, and which should be recyclable at the end of the cable life, energy cables have been recently developed having a cable core made from thermoplastic materials, i.e. polymeric materials which are not crosslinked and thus can be recycled at the end of the cable life.
In this respect, electrical cables comprising at least one coating layer, for example the insulation layer, based on a polypropylene matrix intimately admixed with a dielectric fluid are known and disclosed in WO 02/03398, WO 02/27731, WO 04/066317, WO 04/066318, WO 07/048422, WO2011/092533 and WO 08/058572. The polypropylene matrix useful for this kind of cables comprises a polypropylene homopolymer or copolymer or both, characterized by a relatively low crystallinity such to provide the cable with the suitable flexibility, but not to impair the mechanical properties and thermopressure resistance at the cable operative and overload temperatures. Performance of the cable coating, especially of the cable insulating layer, is also affected by the presence of the dielectric fluid intimately admixed with said polypropylene matrix. The dielectric fluid should not affect the mentioned mechanical properties and thermopressure resistance and should be such to be intimately and homogeneously mixed with the polymeric matrix.
Moreover, for some applications, it is required to provide energy cables in the medium and high voltage range having a cold-strippable semiconductive layer, i.e. an outer semiconductive layer which can be removed during cable installation and jointing without applying heat (the heating procedure requires the presence of further apparatus in situ and may cause damages to the cable) and without challenging the integrity of the underlying layer or leaving residues thereon.
The capability of being cold-strippable should not be detrimental for the adhesion between the semiconductive layer and the insulating layer, since a close and stable bonding between those layers during the cable lifespan prevents partial delamination of the layers with possible formation of micro-voids, which could cause the insurgence of partial discharge phenomena.
US 2006/0182961 (Dow Chemical) relates a semiconductive power cable composition comprising a mixture of (a) a high temperature polymer and a soft polymer, and (b) a conductive filler wherein a semiconductive cable layer prepared from the composition strippably adheres to a second cable layer. Suitable high-temperature polymers include polypropylenes. The high temperature polymer is preferably in the composition in an amount less than 50 weight percent. Suitable soft polymers include polyethylenes and polypropylenes. Polyethylenes include copolymer of ethylene and an unsaturated ester such as a vinyl ester (e.g., vinyl acetate). Suitable polypropylenes include copolymers of propylene and other olefins. A curing agent may be present in the semiconductive composition. The semiconductive polymer base material does not contain a dielectric fluid.
WO 2013/120582 teaches that common concept for making a semi-conductive layer strippable is to increase the polarity of the semi-conductive layer. This document relates to a semi-conducting shield layer of a wire or cable comprising: (A) an ethylene copolymer comprising polar co-monomer units; (B) an olefin copolymer; and (C) a conductive filler; wherein the olefin homo- or copolymer (B) has a degree of crystallinity below 20%. The olefin copolymer (B) can be an ethylene-propylene copolymer. The co-polymer (B) is preferably present in the composition in an amount between 5 and 25% based on the total weight of the polymer composition. The polar co-monomers are selected from the group consisting of acrylic acids, methacrylic acids, acrylates, methacrylates, and vinyl esters. The amount of ethylene copolymer comprising polar co-monomer units is from 30 to 75 wt % of the total polymer composition. The semiconductive polymer base material does not contain a dielectric fluid.